A sulfur vulcanizable copolymer obtained by copolymerization of 1-hexene with a small amount (typically 3-5 mol percent) of methylhexadiene, which may be either a mixture of 4-methyl-1,4-hexadiene (4-MHD) and 5-methyl-1,4-hexadiene (5-MHD), or 5-MHD in substantially pure form, yields on vulcanization a rubber having outstanding biocampatibility and very high flex resistance, excellent ozone resistance, good compression resistance and high damping. This rubber is made and sold by Goodyear Tire and Rubber Company, Akron, Ohio USA, under the trademark "HEXSYN.RTM.". Because of its outstanding biocampatibility and its flex resistance, this rubber has become the elastomer of choice in biomedical devices, including heart valves and diaphragms for artificial hearts and assist devices, e.g., a left ventricle assist devices (LVAD) and artificial finger joints. Synthesis of such rubbers and their use in biomedical applications is described in Lal et al Journal of Polymer Science: Polymer Symposium 74, 141-164 (1986). As this article points out, 5-MHD is more reactive than 4-MHD and has a reactivity essentially equal to that of 1 -hexene. As a consequence, 5-MHD readily copolymerizes with 1-hexene while 4-MHD is poorly incorporated. This leads to copolymers of non-uniform composition.
Lal et al., "Elastomers and Rubber Elasticity", J. E. Mark and J. Lal, eds., ACS Symposium Series No. 193, pp 171-194, (1982) describe copolymerization of 1-hexene with substantially pure 5-MHD, which is obtained by fractionation of a mixture of 4-MHD and 5-MHD.
Other references describing polymerization of alpha-olefins including 1-hexene with non-conjugated dienes including 5-MHD and mixture of 4-MHD and 5-MHD include U.S. Pat. Nos. 3,933,769 and 3,991,262 to Lal et al (both 1976).
U.S. Pat. No. 3,904,704 to Bryson describes a process for production of non-conjugated diolefins by codimerization of ethylene or other alpha-olefin and a conjugated diolefin at high temperature and pressure in the presence of a catalyst. The process is described with particular reference to production of a mixture of 4-methyl-1,4-hexadiene (4-MHD) and 5-methyl-1,4-hexadiene (5-MHD) from ethylene and isoprene. Ratios of 4-MHD and 5-MHD in this product vary widely depending on conditions used, according to data in the patent. A mixture of 4-MHD and 5-MHD is the diene comonomer for synthesis of "Hexsyn" as disclosed in Lal et al Journal of Polymer Science (1986) cited supra. As Lal et al (1982) and (1986) disclose, it is necessary to fractionate the mixture of 4-MHD and 5-MHD if one desires either monomer in pure form. Fractionation is difficult and time-consuming because the boiling points of 4-MHD and 5-MHD are very close (88.degree.-89.degree. C. and 92.8.degree. C. respectively, as reported by Lal et al). Both Lal et al articles also disclose that a diene monomer feed of pure 5-MHD is preferable to the mixture for the reasons stated earlier herein. Nevertheless, routes for preparing pure 5-MHD have not gained acceptance.
Synthesis of a wide variety of hydrocarbons by nickel phosphine complex-catalyzed Grignard coupling, i.e., by cross-coupling of a alkyl, aryl or alkenyl Grignard reagent with an aryl or alkenyl halide, is disclosed in Tamao et al, Bulletin of the Chemical Society of Japan, 49(7), 1958-1969 (1976). Numerous specific reactions are disclosed but none relate to the production of a non-conjugated diolefin and the only reaction for preparing a conjugated diolefin is for the preparation of 2,3-dimethyl-1,3-butadiene from 2-bromo-1-propene and isopropylmagnesium bromide. Yields reported by the authors vary widely, from less than 5% to 100%. Preparation of 2,3-dimethyl-1,3-butadiene from 2-bromo-1-propene and isopropylmagnesium bromide according to Tamao et al was carried out for 20 hours at reflux, with a 79% yield as determined by gas chromatography (GC).
Although Grignard coupling reactions have been known for years, no one has use such reactions commercially for preparation of a non-conjugated diene hydrocarbon as far as the inventors are aware. In fact, as far as the inventors are aware no one has prepared pure 5-MHD free of contamination with structural isomers, by any synthetic route. There remains a need for a process for producing 5-MHD without co-producing appreciable quantities of isomers, in order to provide essentially pure 5-MHD as a diene monomer for the production of polyolefin/diene rubbers.